How to get delivery document no from SAP ByDesign Outbound Delivery?

I have been trying to fetch the Delivery DocNo of an Outbound Delivery. I have set up a Before-save event on Business Object "OutboundDelivery". However, I have successfully fetched the location code of the Outbound Delivery, using the following code. var locationID = this.ShipFromLocation.GetFirst().LocationID.content;

Now I want to get the delivery document no of the delivery but I am not getting it

Can anyone please help me on this?

Below is my code :

import AP.Common.GDT;

import AP.LogisticsExecution.Global;

[Extension] businessobject AP.LogisticsExecution.Global:OutboundDelivery

{ // You must activate this business object before you can access the extension fields // or messages in script files, forms, and screens. node Item { node ItemProduct { node ItemProductSerialNumber { } } node Material { } } }